Shock absorbing fender

ABSTRACT

A marine fender is disclosed which protects a vessel and a dock from damage caused by an impact. The fender is in the form of a tube with a substantial thickness, is comprised of polyurethane and has a uniform arrangement of cavities or openings in its outer surface. The cavities are formed by a web of walls and have substantial depth but do not penetrate through the thickness of the tube to the interior. The fender may also be comprised of four layers. The first layer is comprised of a Shore A hardness of between 80 and 90 and is approximately 25 mm thick. The second layer is comprised of a Shore A hardness of between 75-80 and is approximately 50 mm thick. The third layer is comprised of a Shore A hardness of between 55-75 and is approximately 50 mm thick. The fourth layer is comprised of a Shore A hardness of between 80 and 90 and is approximately 25 mm thick.

This invention relates to a shock absorbing fender formed ofpolyurethane material, and also describes herein, a method and apparatusused to manufacture such a polyurethane shock absorbing device. Inparticular it relates to a shock absorbing device of substantiallycylindrical or tubular construction.

BACKGROUND OF THE INVENTION

Shock absorbing devices of cylindrical construction which are used forprotection, find applications in a wide variety of situations. At oneextreme of size are marine fenders which are employed to minimize thepossibility of damage to wharves and ships during docking procedures, orin heavy seas. At the other extreme of size are shock absorbingcomponents utilised in machines and instrumentation. The aim of thisinvention is to produce a fender having stable operative characteristicsand large energy absorbing ability, relative to its mass and size,coupled with a maximum reaction force when compressed over its designeddeflection characteristics which do not exceed the strength of thesurfaces or members being protected.

The use of marine fenders to protect ships, wharves, drilling rigs andsimilar marine structures is well known. Typically these are ofsubstantially cylindrical or tubular construction and may be ofcircular, D, trapezoid or rectangular cross-section. Various otherdesigns have been employed including inflatable fenders and floatingfenders.

Typically tubular fenders are comprised of rubber material or inparticular styrene butadiene rubber (SBR). In addition some fenders areformed with metal or hard plastic sections or inserts to provideadditional durability, toughness and means of mounting.

Tubular fenders are usually designed to absorb energy by axial or radialelastic compression. The majority of fenders loaded axially arecontained within or attached to complex rigid structures or havesophisticated mounting requirements and shapes to handle largedeflections which are desirable to minimise the reaction force, whichbecomes critical when cushioning larger vessels especially those above150,000 tons, as their steel plate thickness does not increase in directproportion to their mass. Thus their cost effectiveness becomes lesswith increasing size. In addition any of these mounting structures whichhave a considerable inertial mass which is added to the inertial mass ofthe rubber fender further increases the reaction force to a degree wherethe vessel's hull is damaged especially where the closing velocity ishigh.

Similarly prior art tubular fenders which are compressed radially andhaving other than a substantially circular cross section also containcomplexities which reduce their cost effectiveness. Furthermore as thesemore complex shapes need to be molded as monolithic rubber members formaximum effectiveness and durability there is a practical limitation totheir unit size and mass dictated by technological and tooling costconsiderations.

Conversely currently used rubber tubular fenders which are compressedradially and having substantially circular cross-sections may bemanufactured by a process whereby a strip of uncured rubber is woundaround a mandrel until the desired diameter is reached. This laminationis then contained, and cured with heat and pressure. This allows for themanufacture of very large fenders weighing up to 15 tons and costingtens of thousands of dollars. And although their energy absorption perunit mass may not be as efficient as smaller more complex shapes theirrelatively lower manufacturing maintenance and mounting costs sees theirincreasing use, even in smaller sizes, typically of 0.4 m O.D. and 0.2 mI.D. where they are installed in lengths secured to docksides or vesselsby wires or chains threaded through their hollow cores. Even so it willbe appreciated that the larger items are expensive and both labour andmaterial intensive to produce and difficult to handle.

In addition the above fenders commonly of substantially cylindricalconstruction with a hollow core may be supported by a member or memberspassed through the hollow core and each end is attached to the marinestructure. The fender is thus slung against the side of the structure. Acommon support member is a semi-elliptical metal rod supported bychains.

These fenders have different operative characteristics depending on thedegree of compressive load to which they are subjected. For low loads,the amount of energy absorbed may be a linear function of the radialdeflection of the fender surface and the Shear Modulus (G) of the rubberwhich is dependant on the IRHD of the rubber. For thin sections theload-deformation behaviour has been derived by considering the bendingmoment that exists at any cross-section. For thick sections the shearingforces and normal forces must be considered. In use the hollow core ofthe fender may be flattened. At the point inner surface defining ahollow core has been totally compressed the energy absorbingcharacteristics change to those of a solid pad under compression and theIRHD of the bulk material determines the reaction force. Thus for thistype of fender the best performance is achieved where the designedfenders absorb the energy of impact before reaching the limit whereinthe characteristics are those of the bulk material.

The publication titled "Theory and Practice of Engineering with Rubber"which has a Library Congress catalogue card number 78-325872 gives acomprehensive outline of rubber design and calculation principles andspecifically pages 146 to 165.

With respect to the radial compression of long hollow substantiallycircular cylinders where the ratio of the outside diameter (O.D.)divided by the internal diameter (I.D.) is generally less than 2.5, andreferring to the above publication page 148 onwards and applying generalengineering principles with respect to bending stresses in curved beams,it can be appreciated that the maximum fiber stress due to bendingmoments occurs at the diametric plane normal to the applied force.Provided that the cross-section is regular, and the Shear Modulus isessentially constant over the curved section then the fiber stressvaries from a maximum compressive stress at the internal surface to zeroat the neutral axis to a maximum tensile stress at the external surface.For this situation the stress distribution is of a hyperbolic nature andthe neutral axis is located at a radius other than the radius of thecentroid axis. In this situation the neutral axis is located between thecentroid axis and the center of curvature; this always occurs in regularsectioned beams of constant material strength. Of course this may not bethe case if the sectional area or material strength varies in a radialdirection.

Furthermore it can be shown by calculation that for a symmetricalsection the maximum bending stress will always occur at the inside fibersurface of the fender. Calculations show that for a fender of O.D.divided by I.D.=2 this bending stress at the inner surface isapproximately 25% higher than that at the outer surface.

It is also evident that for a constant reaction force supplied by afender the discussed bending stresses applied to the fender fibersincrease as the ratio of the O.D. divided by the I.D. decreases. Forexample decreasing the ratio from 2 to 1.5 doubles the bending stresses.

It is an object of this invention to provide a shock absorbing marinefender which provides protection against damage due to impact, and whichis simpler to produce and easier to handle than existing devices. It isa further object to provide shock absorbing devices of this invention tohave higher lead bearing capacity per unit weight (compared withexisting products), and resistance to tear and cut propagation, thanexisting products.

In this invention a suitable shock absorbing fender can be produced frompolyurethane elastomer material. Such fenders perform at least as wellas rubber or SBR equivalents and in many aspects are far superior.

BRIEF SUMMARY OF THE INVENTION

According to one form of this invention there is provided a shockabsorbing fender comprising polyurethane material, said shock absorbingfender being of a substantially tubular construction and having an outersurface, an inner surface, and two end surfaces said inner surfacedefining a hollow core through which a supporting member may pass, theouter surface having a number of cavities which divide the outer surfaceinto a grid.

Previously the use of polyurethane in large shock absorbing devices hasbeen limited due to the higher density of solid, high performancepolyurethane materials. This increase in the mass coupled with thehigher cost of the raw material makes it uneconomical to produce shockabsorbing devices of identical design to current rubber devices. It is adiscovery of this invention that by introducing cavities into the outersurface of the shock absorbing device, the total amount of polyurethanematerial can be reduced without seriously affecting the shock absorbingproperties of the device. There is a further discovery of this inventionthat the hardness of polyurethane may be varied between the outersurface and the inner surface in such a way that the shock absorbingproperties of the device are not degraded, but the total amount ofpolyurethane material used is reduced. The net effect is a polyurethaneshock absorbing device which has a number of advantages over rubberequivalents.

Polyurethane has a number of advantages for use in shock absorbingdevices. Some of these are that the devices can be moulded from liquidfeedstock which gives considerably more control than existing hotmoulding techniques employed with current rubber devices. The durometerhardness and toughness of the polyurethane can be readily varied duringmanufacture by varying the composition and proportion of the precursormaterials. It can be cast to be completely free of voids, defects anddelaminations or alternatively it can be cast as lower densitymicrocellular foam. Furthermore the cure is complete and even.

Properly formulated polyurethane materials are not susceptible tosurface stress cracking or flex-fatigue in the way that other currentlyused rubbers are when attacked by oxygen and ozone. The materials aretherefore more resistant to the effects of weathering, than arecurrently used rubbers, and can be expected to have a longer life inmost applications.

The preference the cavities are formed by a web of walls which aregenerally of similar cross-sectional width and the depth of which is asubstantial proportion of the total thickness of the cylindrical wall.

By forming a plurality of cavities in the outer surface the total volumeof polyurethane in a shock absorbing device is reduced. This bothreduces the total weight of the device and reduces the cost of itsproduction.

If all other things remained unchanged, the introduction of cavities toa shock absorbing device would reduce its lead bearing capacity. Morespecifically the ability of the device to absorb and dissipate thekinetic energy of an impact is reduced. The benefit of the polyurethanematerials is that this loss of load bearing capacity can be compensatedfor by increasing the formulation hardness of the inner and outersurfaces without a major deterioration of other properties. An addedbenefit is that by increasing the hardness the abrasion and cutresistance of the surface of the device is improved and its coefficientof friction is reduced. By referring to the hardness it is meant theShore A hardness as measured by the indentation test.

By increasing the hardness of the material the Shear Modulus isincreased and the energy absorbing properties of the device, underconditions of compression when the central void has collapsed, aredegraded and the reacting force rises abruptly.

This problem is overcome by reducing the hardness of the bulk materialin layers between the inner and outer surfaces. Such variation ofhardness is not as readily achievable in the manufacture of currentrubber devices. The polyurethane is formulated to provide the highesthardness in layers forming the inner and outer surfaces where thehighest fibre stresses are and where the most arduous conditions exist.The lowest hardness is in a layer formed at the neutral axis of thedevice. The neutral axis is that radial distance from the center ofcurvature within the curved rubber section undergoing stress whereby thetensile and compressive fibre stresses approach zero.

Accordingly, in a further feature of this invention there is provided ashock absorbing device in which the polyurethane material is of adifferent hardness at a first radial distance from the inner surface ascompared to another radial distance from the inner surface.

In preference there can be provided a skin effect so that an externalsurface is made and formed from a polyurethane material which hassubstantial resistance or is in effect a harder material while thematerial other than the outer skin is of a softer type providingtherefore more potential for energy absorption, especially when thecentral bore is closed. Furthermore it is an advantage of the method ofproduction of a shock absorbing fender of this type that fibres can beintroduced into layers during moulding. Therefore, the outermost layercan be of fibre reinforced polyurethane material which has considerableadvantages in resisting weathering. A protective coating of polyurethanemay also be provided to further enhance weatherability.

As it is possible for water and brine to collect in the cavities,accelerated deterioration of the polyurethane in some environments mayoccur. To alleviate this problem drain holes can be provided in thebottom of each cavity. Alternatively, cavities may be provided which areboth inwardly open and outwardly open.

For production of fenders which incorporate this invention there isprovided an apparatus for the production of a shock absorbing device ofsubstantially polyurethane material: comprising a substantially hollowcylindrical drum part adapted to rotate about a cylindrical axis, saiddrum having a profiled inner surface comprising a plurality of inwardlyprojecting bosses adapted to produce cavities in a liquid materialintroduced to the cylinder; a support means adapted to support thecylindrical drum such that the cylindrical axis is substantiallyhorizontal during rotation; means to produce rotation about thecylindrical axis; means to control the speed of rotation of thecylindrical drum; a means of introducing liquid material into thecylindrical drum; and means to maintain a temperature of said liquid.

In preference the means of introducing liquid material into the rotatingcylinder comprises a channel containing a plurality of pouring spoutswhose number and axial positions correspond to internal circumferentialchannels formed between the plurality of internal bosses located in therotating mould, said plurality of internal bosses corresponding inradial and axial position to the plurality of outwardly opening cavitieslocated on the surface of the shock absorbing device.

The inwardly projecting bosses have a dimension and shape correspondingto the plurality of outwardly opened cavities contained within andextending radially inwards from the external cylindrical surface of theshock absorbing device such that their relationship is male to femalewithin the terms of mould and pattern-making.

In preference the bosses are attached to essentially flat longitudinalbars which are movable either in a radial or axial direction withrespect to the cylindrical drum to allow withdrawal of the completedshock absorbing device from the cylindrical drum.

In preference the cylindrical drum consists of a rigid hollow cylinderslightly larger in length and diameter than the dimensions of the marinefender to be produced to allow for the space taken up by said flatlongitudinal bars.

Alternatively the bosses may be integrally formed with the longitudinalbars. The bars are longer than the fender to be cast but shorter thanthe rotating cylinder. A number of bars are distributed around the innersurface of the drum with a long axis of the bar being parallel to therotating axis. The width and number of the bars is such that theirlongitudinal edges closely abut when affixed to the inner surface of thedrum.

In preference the longitudinal bars are semi-rigid and capable offlexure, being preferably attached to the drum at each end.

The flexure of the bar aids in the stripping and removal from the drumof the shock absorbing device after moulding. There is also provided aslight arch in the bar which prevents vibrational damage at lowrotational speeds when the drum is empty.

In preference there is provided a rigid removable plate or flangeadapted to affix to the open ends of the hollow cylindrical drum toprevent liquid from leaking from the drum. These plates have a centralcircular open portion through which the means of introducing liquid tothe drum enters.

There is also preferentially provided a means of circulating airinternally through the cylindrical drum.

In the moulding of a fender according to this invention there isprovided a method of producing a polyurethane shock absorbing devicehaving a plurality of cavities comprising the steps of mixing apolyurethane reaction mixture in predetermined proportions andintroducing the mixture into a cylindrical drum rotating at apredetermined number of revolutions per minute; rotating the drumcontaining the mixture until such time as the polyurethane materialreaches a tack phase; introducing a further mixture of the same ordifferent composition and repeating the procedure a number of times,each time producing an additional layer, until a shock absorbing deviceof the appropriate dimensions is produced.

A tack phase is a stage in the curing of the introduced liquidpolyurethane material at which the surface is partly cured. Addition ofadditional material at this stage produces a bond between layers ofsubstantial strength.

The elastomeric polyurethane is produced from a liquid polyurethanereaction mixture which is capable of solidifying by additionpolymerization into an elastomeric material. This mixture may beprepared by the one shot, quasi-prepolymer or prepolymer methods usingcommercially available dispensing and mixing machines. The primaryprecursors are isocyanates, polyols and diols. Secondary precursors suchas carbon pigments, anti-foaming agents or blowing agents can also beincluded to produce desired characteristics.

In particular the polyurethane reaction mixture is obtained frompredominantly difunctional hydroxyl terminated compounds having averageequivalent weights of 500 to 1500 and low average functionalitypolyisocyanate compounds. The isocyanate compounds preferably having afunctionality less than 2.4.

The liquid polyurethane reaction mixture is capable of modification toproduce specific variations in physical and chemical end properties ofthe solidified elastomeric material.

The modifications may include: varying the hydroxyl terminated compoundas to its chemical composition; using two or more hydroxyl terminatedcompounds in various ratios; varying the processing conditions prior toor during final mixing; varying the order in which the various reactantsare mixed; varying the functionality or equivalent weight of thehydroxyl terminated compounds; using two or more chemically differentisocyanate compounds; varying the functionality of the isocyanatecompounds; varying the stoichiometry of the reactants away from atheoretically balanced ratio.

The specific variations in properties of the reactant mixture andsolidified elastomer may be further affected by the addition ofdifunctional chain extenders preferably having an equivalent weight of150 or less. In addition, and always in combination with a chainextender, small quantities of trifunctional or polyfunctional crosslinkers may be added to the reactant mixture.

The cross linkers are preferably less than 10% by weight of the chainextender compounds.

The chain extender and cross linker compounds may be added to any of thepreviously mentioned reactant compounds either prior to or during thefinal reactant mixing operation.

Also certain compounds known in the field of polyurethane technology asadditives may be blended with any of the previously mentioned reactantcompounds, or any combination of these previously mentioned compoundseither prior to or during final mixing of the polyurethane reactionmixture. The additive compounds may be reactive or non reactive towardsany of the compounds previously mentioned as comprising the polyurethanereaction mixture.

A comprehensive list of additives and their uses is outlined in apublication entitled, "The Development and Use of Polyurethane Products"which has a Library of Congress catalogue card number 71-141918. Thispublication also contains a list of reactants which will combine withisocyanates.

To aid in the understanding of this invention preferred embodiments willbe described with the aid of the following drawings, in which:

FIG. 1 is a perspective view of a first embodiment of a cylindricalshaped shock absorbing device of circular cross-section having aplurality of cavities;

FIG. 2 is a cross-sectional side view through the line 2--2 of the shockabsorbing device of FIG. 1;

FIG. 3 is a cross-sectional end view through the line 3--3 of the shockabsorbing device of FIG. 1; showing the radial variation in hardness;

FIG. 4 is a cross-sectional end view as in FIG. 3 showing the radialvariation in hardness; and

FIG. 5 is a perspective view of a second embodiment of a cylindricalshaped shock absorbing device with a polygonal outer surface having aplurality of cavities.

Referring to the drawings, there is provided a shock absorbing device 1of circular cross-section, having an outer surface 2, an inner surface 3and two end surfaces 4. The inner surface 3 defines a hollow core 5through which a support member may pass. Formed in the outer surface 2are a number of outwardly open cavities 6 which are defined by anintervening wall system 7. The surfaces 2, 3, 4 and surfaces delineatingthe recesses are thus defined by skin parts of the marine fender. Forexample this device may have an outer radius of 900 mm, an inner radiusof 450 mm and a total length 2500 min.

The cavities 6 are essentially square in this embodiment but may be ofvirtually any shape.

There are provided drain holes 6a in the cavities to allow water andbrine to drain from the cavities which face upwards. In the absence ofdrain holes the shock absorbing device may experience accelerateddeterioration. To further aid in the weather resisting properties of thedevice the outer layer may be formulated with fibrous reinforcement.

The device is produced by casting a first layer 8 with Shore A hardnessof between 80 and 90. This layer is approximately 25 mm thick. A secondlayer 9 of 50 mm thickness is cast with a hardness of 75-80 A. This isfollowed by a third layer 10 which is also approximately 50 mm thick andhas a hardness of 55-75 A. Either or both of the second and third layersmay be microcellular. Finally an innermost layer 11 is cast of 80-90Shore A hardness and approximately 25 mm thickness.

A preferred formulation consists of: mixed glycol polyols based oncombinations of ethylene and/or diethylene and/or butylene-adipate ofequivalent weight of 1000; 1,4 Butane diol and for harder formulationssome trimethyl propane; and 4,4 Diphenylmethane di-isocyanate. Thehardness may be varied by altering the ratio of polyol to diol. For asoft formulation with approximately Shore A hardness of 45 no diol isused and the reactants are mixed in the proportion: 1 equivalent weightof Polyol: 1 equivalent weight of Isocyanate.

For a hard formulation with approximate Shore D hardness of 50 thereactants are mixed in the proportion:

1 equivalent weight of Polyol: 5 equivalent weight of 1,4 Butane Diol: 6equivalent weight of Isocyanate

The exact hardness and properties depends on the variations inprocessing and amount of the trimethyl propane used to replace some 1,4Butane Diol, and any slight variation on the NCO percentage. Obviouslyother hardnesses may be obtained by using other ratios of polyol todiol.

A second embodiment 12 varies from the first in that the outer surface13 is of a polygonal shape. The cavities 14 are similar to those of thefirst embodiment.

I claim:
 1. A shock absorbing fender comprising a tubular elastomericdevice having two end surfaces,an inner surface extending between theend surfaces and defining a hollow core, an outer surface extendingbetween the end surfaces and surrounding the inner surface, a pluralityof cavity defining surfaces extending inwardly from said outer surfaceat least partway to the inner surface, forming cavities separated bycavity walls which divide the outer surface into a grid of intersectingouter surface wall portions, said cavity defining surfaces of each saidcavity including circumferentially spaced surfaces, which are generallyparallel, such that an adjacent said wall portion has a thickness whichdecreases towards said inner surface.
 2. A shock absorbing fenderaccording to claim 1 wherein said unitary polyurethane elastomercomprises two radially spaced relatively hard layers and a relativelysoft layer, said inner and outer surfaces being surfaces of therelatively hard layers and the relatively soft layer being an innerlayer lying between the relatively hard layers.
 3. A shock absorbingfender comprising a tubular device of monolithic polyurethane elastomerhaving two end surfaces,an inner surface extending between the endsurfaces and defining a hollow core, an outer surface extending betweenthe end surfaces and surrounding the inner surface, a plurality ofcavity defining surfaces extending inwardly from said outer surfacepartway to the inner surface forming cavity walls each of which, incross-section, has two pairs of generally parallel walls, said cavitiesdividing the outer surface into a grid of intersecting outer surfacewall portions, said monolithic polyurethane elastomer comprising fourradially spaced layers, said inner and outer surface being surfaces ofinner and outer of said flow layers which are relatively hard, a thirdof said four layers, contiguous with said inner layer, being relativelysoft, and a fourth of said layers, between said third and outer layers,having an intermediate hardness which is between the hardness of saidthird and outer layers.
 4. A shock absorbing fender according to claim 3wherein each said cavity extends radially inwardly approximately twothirds of the distance between said outer and inner surfaces.
 5. A shockabsorbing fender according to claim 3 further comprising drain holesextending from respective cavities to said inner surface.
 6. A marinefender for use in protecting a vessel and a dock from damage caused byimpact with one another comprising:a) an elongate, tubular, elastomericbody having spaced ends and generally circular and concentric elongateinner and outer surfaces extending from one end to the other, the innersurface defining a hollow core and the outer surface surrounding theinner surface; b) the body including four radially spaced layers betweenthe inner surface and the outer surface, each extending from one end tothe other; c) the inner and outer surfaces being surfaces of inner andouter of said layers which are relatively hard; d) a third of said fourlayers, contiguous with said inner layer, being relatively soft; and e)a fourth of said layers, between said third and outer layers, having anintermediate hardness which is between the hardness of said third andouter layers.
 7. The fender of claim 6 wherein said third and fourthlayers are formed of microcellular material.
 8. The fender of claim 6wherein the geometric configuration of the tubular body is normallycylindrical.
 9. The fender of claim 6 wherein the geometricconfiguration of the tubular body is normally polygonal.
 10. A marinefender for use in protecting a vessel and a dock from damage caused byimpact with one another comprising:a) an elongate, tubular, elastomericbody having spaced ends and generally circular and concentric elongateinner and outer surfaces extending from one end to the other; b) thebody including radially spaced, relatively hard inner and outer tubularportions respectively adjacent and including skin parts defining theinner and outer surfaces, the inner and outer portions each extendingfrom one end to the other; c) the body also including an elongate,tubular, relatively soft, central portion interposed between and infused connection with the inner and outer portions whereby to provide aunitary tubular fender having high radial compressibility and resistanceto ambient condition induced degradation; d) wherein said body includesa plurality of generally radially disposed recesses projecting inwardlyfrom outer openings near an imaginary geometric configuration generatedby the outer surface and corresponding to an overall configuration ofthe body generated by the outer surface; and e) wherein a plurality ofrecess defining surfaces extend inwardly from said outer surface atleast partway to the inner surface forming said recesses separated byrecess walls which divide the outer surface into a grid of intersectingouter surface wall portions, said recess defining surfaces of saidrecess including circumferentially spaced surfaces which are generallyparallel, such that an adjacent said wall portion has a thickness whichdecreases towards said inner surfaces.
 11. The fender of claim 10wherein at least some of the recesses are through holes each extendingfrom its said opening to an inner surface opening.